As a compact short wavelength laser or a wavelength conversion laser, there is an OPO (Optical Parametric Oscillator) laser representative of an SHG (Second Harmonic Generation) laser using a non-linear optical crystal. This laser has a laser beam of a prescribed wavelength as a basic beam, converts the basic beam into a laser beam having a wavelength different from the wavelength of the basic beam and outputs the converted beam. The laser that performs the above-described wavelength conversion is used as various kinds of light sources of an optical disc device or an optical communication module or the like.
Such a laser employs a method that a solid-state laser crystal is provided in an internal cavity type OPO laser in which a non-linear optical crystal is disposed in a cavity and a laser beam outgoing from a semiconductor laser is allowed to be incident on the end face of the solid-state laser crystal side to pump the solid-state laser crystal and generate the basic light.
In this case, the above-described laser comes into contact with or sticks to all optical elements disposed in the cavity to form the cavity by the end face of the input side of the laser beam and the end face of the output side of the laser beam. Thus, a device can be made compact. Further, the optical elements may not need to be adjusted, the device can be applied to various kinds of uses. Further, not only the above-described laser can be made compact, but also conversion efficiency can be improved by repeatedly reflecting the basic beam in the cavity.
In a method for producing an optical element used in such a laser, relatively large wafer type two crystal parts are stuck to each other by an adhesive, and then, an optical film or the like is formed and the obtained product is cut into chips.
In this method, since the wafers can be stuck to each other under a state that the warp of the wafers due to the stress of the optical film is not produced, an adhesive layer having a uniform thickness can be obtained. Accordingly, not only various types of characteristics of optical parts can be improved, but also unevenness in characteristics of the cut chips can be suppressed.
In the above-described method for producing the optical element, the adhesive layer is exposed to high temperature upon formation of the optical film. Thus, the adhesive layer may be possibly peeled off or the damage of the crystal parts may be caused due to the difference in coefficient of linear expansion between the two crystal parts.
In the laser for generating the basic beam under the pumping by the laser beam from the semiconductor laser, the laser beam heats the optical elements. Accordingly, when the output of the laser beam is raised, the adhesive layer is peeled off or the two crystal parts are broken owing to the difference in coefficient of linear expansion between the two crystal parts.
Especially, when the two crystal parts greatly different in coefficient of linear expansion are stuck to each other, the peel-off of the adhesive layer or the damage of the crystal parts is outstandingly generated.
As a cause that the above-described adhesive layer is peeled off or the crystal parts are broken, a below-described phenomenon is considered. When the two crystal parts greatly different in coefficient of linear expansion are stuck to each other and exposed to high temperature under a condition that the two crystal parts are stuck to each other, a stress is generated in the adhesive layer due to the difference in coefficient of linear expansion. When the adhesive layer mitigates this stress, the adhesive layer will not be peeled off, nor a base will be broken. When the adhesive layer cannot mitigate the stress, the peel-off or the damage will be firstly generated from parts whose strength is lowest. Whether or not the adhesive layer is separated from the crystal parts or whether or not the crystal parts are broken is determined depending on the relation between the adhesive strength of the adhesive layer and the brittleness of the crystal parts. Here, whether or not the adhesive layer can mitigate the stress is determined depending on the mechanical materiality of the adhesive layer in which the adhesive is cured and a geometric factor including the thickness of the adhesive layer and an adhesive area.